The present invention relates to a cathode ray tube, and in particular to a color cathode ray tube having an electron gun employing a multistage focus lens for focusing a plurality of electron beams on a phosphor screen.
Shadow mask type color cathode ray tubes are prevailingly used as TV picture tubes and monitor tubes for information terminals. The shadow mask type color cathode ray tubes house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which serves as a color selection electrode and is closely spaced from the phosphor screen. The electron beams emitted from the electron gun are deflected to scan the phosphor screen two-dimensionally by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and to display a desired image on the phosphor screen.
FIG. 17 shows a cross-sectional view for explaining an example of the constitution of the shadow mask type color cathode ray tube, and in FIG. 17, reference numeral 201 denotes a panel portion forming a viewing screen, 202 denotes a neck portion for housing an electron gun, 203 denotes a funnel portion for connecting the panel portion and the neck portion, 204 denotes a phosphor screen, 205 denotes a shadow mask serving as a color selection electrode, 206 denotes a mask frame for supporting the shadow mask 205, 207 denotes a magnetic shield for shielding extraneous magnetic fields such as the earth""s magnetic field, 208 denotes a mask suspension mechanism, 9 denotes an in-line type electron gun, 10 denotes a deflection yoke, 11 denotes an internal conductive coating, 12 denote stem pins, and 13 denotes a getter.
In the case of the color cathode ray tube, the evacuated envelope is comprised of the panel portion 201, the neck portion 202 and the funnel portion 203, and electron beams B (one center electron beam and two side electron beams) emitted from the electron gun 9 housed in the neck portion 202 scan the phosphor screen 204 in two dimensions by the horizontal and vertical direction magnetic fields produced by the deflection yoke 10.
The deflection yoke 10 is of a self-converging type which provides a pin cushion-like horizontal deflection magnetic field and a barrel-like vertical deflection magnetic field to converge a plurality of electron beams over the entire phosphor screen.
The electron beams B are modulated in amount by modulating signals such as video signals supplied via the stem pins 12, are color-selected by the shadow mask 205 disposed immediately in front of the phosphor screen 204, and impinge upon the phosphors of the corresponding colors to reproduce a desired image.
The cathode ray tubes of this kind are provided with a multistage focus lens in the electron gun and a dynamic focusing system is widely adopted where at least one of the electrodes constituting the multistage focus lens is supplied with a voltage varying dynamically, to obtain sufficiently small electron beam spots over the entire phosphor screen.
FIG. 18 is a schematic cross-sectional view of an example of an electrode structure of an electron gun employed in a color cathode ray tube, taken perpendicular to the in-line direction of three in-line electron beams.
In FIG. 18, reference numeral 1 denote three cathodes each having a heater incorporated therein, and electron beam generating means comprises the cathodes 1, a first electrode 2 serving as a control electrode and a second electrode 3 serving as an accelerating electrode and the electron beam generating means forms electrons generated by the three cathodes 1 into three respective electron beams. Electron beam focusing means comprises a third electrode 4, a fourth electrode 5, a fifth electrode 6 and an anode 7, and the electron beam focusing means accelerates the three electron beams and focuses them on the phosphor screen 204. Reference numeral 8 denotes a shield cup and Eb is an anode voltage. The fifth electrode 6 is divided into a first member 61 and a second member 62.
The third electrode 4, the fourth electrode 5 and the first member 61 of the fifth electrode 6 form a first-stage focusing lens, and the second member 62 of the fifth electrode 6 and the anode 7 form a second-stage focusing lens.
The electrons emitted from the cathodes 1 heated by the heaters are accelerated toward the first electrode 2 serving as an electron beam control electrode by an accelerating potential of the second electrode 3 to form three electron beams. After passing through the electron beam apertures in the second electrode 3 and the third electrode 4, the three electron beams are slightly focused by the first-stage focusing lens formed by the third electrode 4, the fourth electrode 5 and the first member 61 of the fifth electrode 6.
After passing through the first-stage focusing lens, the electron beams enter the second-stage focusing lens formed by the second member 62 of the fifth electrode 6 and the anode 7 and serving as a main lens.
In FIG. 18, reference numeral 63 denotes a correction plate electrode disposed within the second member 62 of the fifth electrode 6 and 71 is a correction plate electrode disposed within the anode 7.
The three respective electron beams are focused while they pass through the second-stage focusing lens, then are subjected to color selection by the shadow mask 205, and then are focused on phosphor elements of an intended color of the phosphor screen 204 to form an electron beam spot.
A first focusing voltage Vf1 of a fixed voltage is applied to the third electrode 4 and the first member 61 of the fifth electrode 6, and a second focusing voltage (Vf2+dVf) of a fixed voltage Vf2 superposed with a dynamic voltage dVf varying in synchronism with deflection angle of the electron beams scanning the phosphor screen 204 is applied to the second member 62 of the fifth electrode 6. With this structure, the curvature of the image field is corrected by varying the strength of the main lens according to the deflection angle of the electron beams.
In addition to the above structure, an electrostatic quadrupole lens is formed by four vertical plates 611 attached to the end of the first member 61 of the fifth electrode 6 on the second member 62 side thereof and two horizontal plates 621 attached to the end of the second member 62 of the fifth electrode 6 on the first member 61 side thereof. With the electrostatic quadrupole lens being configured so as to focus the electron beams horizontally and so as to diffuse the electron beams vertically according to increasing deflection angles of the electron beams, the electrostatic quadrupole lens corrects astigmatic deflection defocusing induced by the deflection yoke which diffuses the electron beams horizontally and focuses the electron beams vertically according to the increasing deflection angles of the electron beams. With this structure, a good focus is obtained over the entire viewing screen.
But electron guns for use in color cathode ray tubes such as TV picture tubes and display monitor tubes need to control the cross-sectional shape of the electron beams properly according to the amount of electron beam deflection so as to provide a good focus characteristic and high resolution over the entire viewing screen.
With the above electron gun, the cross-sectional shape of the electron beams entering the main lens is elongated vertically according to the increasing deflection angle of the electron beams by the astigmatism-correcting electrostatic quadrupole lens, consequently the vertical diameter of the cross section of the electron beams is influenced greatly by the deflection defocusing which compresses the vertical diameter of the cross section of the electron beams and expands the horizontal diameter of the cross section to elongate the cross section horizontally, and as a result the electron beam spots are elongated horizontally at the periphery of the viewing screen and it was difficult to obtain good and uniform focus over the entire viewing screen.
To eliminate the above problem, in addition to the above electrostatic quadrupole lens, another electrostatic quadrupole lens serving as an electron beam shaping lens is formed by dividing the fifth electrode again or by dividing the third electrode again and is disposed in a position remoter from the anode than the above electrostatic quadrupole lens is.
The additional electrostatic quadrupole lens diffuses the electron beams in the direction in which the anode-side electrostatic quadrupole lens focuses the electron beams and focuses the electron beams in the direction in which the anode-side electrostatic quadrupole lens diffuses the electron beams such that the additional electrostatic quadrupole lens has opposite effects on the electron beams from the anode-side electrostatic quadrupole lens.
With this structure, the electrostatic quadrupole lens for shaping the electron beams can be configured so as to elongate the cross section more horizontally according to the increase in the electron beam deflection and the astigmatism-correcting electrostatic quadrupole lens can shape the cross-sectional shape of the electron beams easily, and consequently good and uniform focus can be obtained over the entire viewing screen.
But there is a problem in that sufficient shaping action of elongating the cross section of the electron beams horizontally cannot be obtained even when the electron beam-shaping electrostatic quadrupole lens is formed within the third electrode remotest from the anode so as to shape the electron beams most effectively, and consequently good and uniform focus cannot be obtained over the entire viewing screen.
It is an object of the present invention to provide a high resolution color cathode ray tube having eliminated the problems of the prior art and optimized the shape of the electron beam spots over the entire viewing screen.
To accomplish the above objects, in accordance with an embodiment of the present invention, there is provided a color cathode ray tube comprising an evacuated envelope comprising a panel portion, a neck portion and a funnel portion for connecting said panel portion and said neck portion, a phosphor screen formed on an inner surface of said panel portion, an in-line type electron gun housed in said neck portion, and an electron beam deflection yoke mounted around said neck portion, said in-line type electron gun comprising an electron beam generating section having a plurality of in-line cathodes, a first electrode serving as an electron beam control electrode and a second electrode serving as an accelerating electrode arranged in the order named for projecting a plurality of electron beams arranged approximately in parallel with each other in a horizontal plane toward said phosphor screen, an electron beam focusing section comprising a third electrode, a fourth electrode, a fifth electrode and an anode arranged in the order named for focusing said plurality of electron beams on said phosphor screen, said third electrode comprising a first group of members and a second group of members of said third electrode, said fifth electrode comprising a first group of members and a second group of members of said fifth electrode, said first group of members of said third electrode and said first group of members of said fifth electrode being supplied with a first focus voltage of a fixed value, and said second group of members of said third electrode and said second group of members of said fifth electrode being supplied with a second focus voltage comprised of a fixed voltage and a dynamic voltage varying with deflection of said plurality of electron beams, wherein at least one first-type electrostatic quadrupole lens is formed between said first and second groups of members of said fifth electrode for increasingly focusing said plurality of electron beams in one of horizontal and vertical directions and for increasingly diffusing said plurality of electron beams in another of the horizontal and vertical directions with an increase in a focus voltage difference between said first focus voltage and said second focus voltage, at least one second-type electrostatic quadrupole lens is formed between said first and second groups of members of said third electrode for increasingly focusing said plurality of electron beams with the increase in the focus voltage difference in a direction perpendicular to said one of the horizontal and vertical directions in which one disposed nearest said anode, of said at least one first-type electrostatic quadrupole lens increasingly focuses said plurality of electron beams with the increase in the focus voltage difference, and for increasingly diffusing said plurality of electron beams with the increase in the focus voltage difference in a direction perpendicular to said another of the horizontal and vertical directions in which said one disposed nearest said anode, of said at least one first-type electrostatic quadrupole lens increasingly diffuses said plurality of electron beams with the increase in the focus voltage difference, and an electron lens is formed between said fourth electrode and a first aperture formed in a first surface of one member of said second group of said third electrode adjacent to said fourth electrode and forming said at least one second-type electrostatic quadrupole lens in combination with one member of said first group of said third electrode, said first surface of said one member of said second group of said third electrode being on a side of said one member of said second group of said third electrode opposite from said fourth electrode, said electron lens being configured so as to increasingly diffuse said plurality of electron beams in the horizontal direction with an increase in a voltage difference between said second focus voltage and a voltage applied to said fourth electrode and to increasingly focus said plurality of electron beams in the vertical direction with the increase in the voltage difference between said second focus voltage and the voltage applied to said fourth electrode.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a color cathode ray tube comprising an evacuated envelope comprising a panel portion, a neck portion and a funnel portion for connecting said panel portion and said neck portion, a phosphor screen formed on an inner surface of said panel portion, an in-line type electron gun housed in said neck portion, and an electron beam deflection yoke mounted around said neck portion, said in-line type electron gun comprising an electron beam generating section having three in-line cathodes, an electron beam control electrode and an accelerating electrode arranged in the order named for projecting three electron beams arranged approximately in parallel with each other in a horizontal plane toward said phosphor screen, an electron beam focusing section comprising a third electrode, a fourth electrode, a fifth electrode and an anode arranged in the order named for focusing the three electron beams on said phosphor screen, said third electrode comprising a first group of members and a second group of members of said third electrode, said fifth electrode comprising a first group of members and a second group of members of said fifth electrode, one member of said second group of members of said fifth electrode being disposed adjacently to said anode, said first group of members of said third electrode and said first group of members of said fifth electrode being supplied with a first focus voltage of a fixed value, and said second group of members of said third electrode and said second group of members of said fifth electrode being supplied with a second focus voltage comprised of a fixed voltage and a dynamic voltage varying with deflection of the three electron beams, wherein at least one first-type electrostatic quadrupole lens is formed between said first and second groups of members of said fifth electrode for increasingly focusing the three electron beams in one of horizontal and vertical directions and for increasingly diffusing the three electron beams in another of the horizontal and vertical directions with an increase in a focus voltage difference between said first focus voltage and said second focus voltage, at least one second-type electrostatic quadrupole lens is formed between said first and second groups of members of said third electrode for increasingly focusing the three electron beams with the increase in the focus voltage difference in a direction perpendicular to said one of the horizontal and vertical directions in which one disposed nearest said anode, of said at least one first-type electrostatic quadrupole lens increasingly focuses the three electron beams with the increase in the focus voltage difference, and for increasingly diffusing the three electron beams with the increase in the focus voltage difference in a direction perpendicular to said another of the horizontal and vertical directions in which said one disposed nearest said anode, of said at least one first-type electrostatic quadrupole lens increasingly diffuses the three electron beams with the increase in the focus voltage difference, and an electron lens is formed between said fourth electrode and an aperture formed in one member of said second group of said third electrode adjacent to said fourth electrode and forming said at least one second-type electrostatic quadrupole lens in combination with one member of said first group of said third electrode, said one member of said second group of said third electrode being a plate-like electrode and said aperture being a vertically elongated aperture, said electron lens being configured so as to increasingly diffuse the three electron beams in the horizontal direction with an increase in a voltage difference between said second focus voltage and a voltage applied to said fourth electrode and to increasingly focus the three electron beams in the vertical direction with the increase in the voltage difference between said second focus voltage and the voltage applied to said fourth electrode.
The invention is not limited to particular details of the above-explained construction and the below-explained embodiments. Various changes and modifications can be made to the above-explained structures and the below-explained embodiments without departing from the spirit and scope of the invention as defined in the appended claims.